There are 2 issues right on new Virtualmin installations on CentOS 7.3
I was able to replicate this by re-installing it on new test box several times.
The errors are related to the initial Wizard.
When you get to the ClamAv selection it fails with an error as described here: https://www.virtualmin.com/node/35862#comment-774036
Then when you get to the MySQL part it asks for the new root password (that is not set yet) and as well execute the secure mysql actions that removes the test database and anonymous users.
When you put in the new password and select to remove the test BD and user it then fails with an access denied to localhost MySQL error. When you hit back it works, and the password is indeed set correctly in MySQL/MariaDB, but now the Wizard shows the next MySQL step to assign the installation type, default, small, etc. When you check the MySQL setting after back in Webmin, some times it correctly deletes the test user and test database, sometimes they seem to be still there. This could be related to the error when you hit back in the wizard.